If you receive an notification or error message similar to the below;

• Your email server rejected your login
• Incorrect password entered
• Username or password incorrect

Have you recently changed your password? 

If so, it could be that you have not yet updated the password on your device.

Note, the password must be setup for both incoming and outgoing settings.

Please refer to this article on how to Modify mail server settings - iOS - Mail

App Specific Passwords (ASP)

Have you recently setup ASP?  If so, you will need to update the password on your device using the ASP.

Note, the password must be setup for both incoming and outgoing settings.

Please refer to this article on how to Modify mail server settings - iOS - Mail